1. Field of the Invention
The present invention relates to a method of processing noise in image data such that at least one of judgment of noise produced at random in time by an imaging device such as a CCD image sensor or CMOS image sensor and reduction of the random noise is enabled. The invention also relates to a noise reduction unit and to an imaging apparatus using the noise reduction unit.
2. Description of the Related Art
In an imaging apparatus (such as a digital camera capable of imaging motion pictures with a solid-state imaging device (e.g., a CCD image sensor or CMOS image sensor)), a camera built in a cell phone, or a digital camera, it is important to reduce noise produced by the solid-state imaging device under low luminance conditions during shooting in order to achieve high image quality.
Noises generated by solid-state imaging devices are generally classified into fixed pattern noise and random noise. It is common practice to suppress fixed pattern noise with a correlated double sampling (CDS) circuit (see “Fundamentals of Solid-State Imaging Devices”, pp. 81-84, Nippon-Rikoh Publishing Society, Japan (non-patent reference 1)). Meanwhile, random noise is produced at random on the time axis and, therefore, it is difficult to suppress the random noise. However, reduction of random noise is important in preventing deterioration of S/N of the solid-state imaging device and in eliminating the detection limit under low luminance conditions, as well as in achieving high image quality.
Where noise is reduced by an autocorrelation method, when periodic flicker noise, fixed pattern noise, or beat noise is produced in the image, it is possible to capture the pattern (feature) of the noise by calculating the autocorrelation coefficient from the image data. In this case, the autocorrelation function is calculated by
                              r          ⁡                      (            τ            )                          =                              1            N                    ⁢                                    ∑                              t                =                0                                            N                -                τ                -                1                                      ⁢                                                  ⁢                                          x                ⁡                                  (                                      t                    +                    τ                                    )                                            ⁢                              x                ⁡                                  (                  t                  )                                                                                        (        1        )            where N is the total number of pixels, x is the pixel value indicating the brightness of the pixel, t is time, and τ is the amount of shift of the pixel along the time axis. The autocorrelation function r(τ) is the arithmetic mean of values of the product of the pixel value at instant of time t and the pixel value at an instant (t+τ) shifted by τ.